Field
The present invention relates to an inorganic semiconductor light emitting device, and more particularly, to an ultraviolet light emitting device.
Discussion of the Background
Generally, a gallium nitride-based semiconductor has been widely used in a blue/green light emitting diode or a laser diode as a light source of full color displays, traffic lighting, general lamps and optical communication instruments. In particular, an indium gallium nitride (InGaN) compound semiconductor has attracted considerable attention due to its narrow band gap.
This gallium nitride-based compound semiconductor has been utilized in various fields such as large-sized natural color flat panel display devices, light sources of backlight units, traffic lights, indoor lighting fixtures, high density light sources, high resolution output systems, optical communication, and the like. A light emitting device for emitting near ultraviolet light has been used in forgery discrimination, resin curing and ultraviolet treatment, and can realize various colors of visible light in combination with a fluorescent substance.
Near ultraviolet light refers to ultraviolet light at wavelengths ranging from about 320 nm to 390 nm. Gallium nitride GaN has an energy band gap of about 3.42 eV, which correspond to optical energy at a wavelength of about 365 nm. Accordingly, a light emitting device including an InGaN well layer can be used to emit near ultraviolet light at wavelengths of 365 nm or greater, that is, wavelengths from 365 nm to 390 nm according to In content.
Since light produced in the well layer is emitted to the outside through a barrier layer and a contact layer, a plurality of semiconductor layers is located in a path along which light travels, and light absorption occurs due to the semiconductor layers. In particular, when the semiconductor layers have a band gap smaller than or similar to those of the well layers, significant light loss occurs. In particular, it is necessary to control light absorption due to an n-type contact layer and a p-type contact layer occupying most of the thickness of the light emitting device.
Thus, in the near ultraviolet light emitting device in the related art, barrier layers, n-type contact layers, and p-type contact layers as well as electron blocking layers are formed of AlGaN which has a greater band gap than InGaN. However, since it is difficult to grow AlGaN relatively thick while ensuring good crystallinity of AlGaN, electric and optical characteristics of the near ultraviolet light emitting device are inferior to those of blue light emitting devices, and the near ultraviolet light emitting device is sold at a higher price than blue/green light emitting devices.
FIG. 5 is a schematic sectional view of a typical light emitting diode and FIG. 6 is an enlarged sectional view of an active area of the light emitting diode of FIG. 5.
Referring to FIG. 5 and FIG. 6, the light emitting diode includes a substrate 111, a three-dimensional growth layer 113, an n-type contact layer 115, an active area 117, a p-type contact layer 119, an n-electrode 110, and a p-electrode 120. In such a typical light emitting diode, the active area 117 having a multi-quantum well structure is placed between the n-type contact layer 115 and the p-type contact layer 119 to improve luminous efficacy and can emit light of a desired wavelength by adjusting the In content of InGaN well layers in the multi-quantum well structure.
GaN has a band gap of about 3.42 eV, which corresponds to energy of light having a wavelength of about 365 nm. Accordingly, a light emitting diode using GaN or InGaN in a well layer, which may improve luminous efficiency due to a difference in band gap between the well layer and a barrier layer, emits blue light or UV light having a wavelength of about 400 nm or more. In order to provide a light emitting diode emitting UV light having a wavelength of 400 nm or less, it is necessary to increase the band gaps of the well layers and the barrier layers. To this end, well layers formed by adding Al to GaN or InGaN are used (see Korean Patent Publication No. 10-2012-0129449A).
In the active area that includes well layers comprised of InGaN and emits light having a wavelength of 400 nm or more, there may be a large difference in band gap between the GaN or InGaN barrier layers and the well layers to provide high quantum efficiency within the well layers. However, in order to improve quantum efficiency in the active area including the well layers having an band gap capable of emitting light having a wavelength of 400 nm or less by adding Al to GaN or InGaN, the barrier layers may have a higher band gap.
Referring to FIG. 6, in the active area 117 of the typical light emitting diode, barrier layers 117b have a greater thickness than well layers 117w. This structure may improve luminous efficiency through maximization of a recombination rate between holes and electrons in the well layers 117w. More specifically, the well layers and the barrier layers are alternatively stacked one above another in at least one pair. When electrons and holes are injected into the well layer and confined therein, each of the electrons and holes may not be regarded as a single particle. That is, the electrons and the holes confined in the well layer are probabilistically present within the quantum well structure according to the probability distribution thereof. The probability distribution of electrons and holes can be represented by a distribution graph by existence probability according to the principle of uncertainty. Accordingly, although the electrons and the holes may be injected into the well layers in the active area, there is a possibility of existence of the electrons and the holes in the barrier layers according to the probability distributions thereof.
Electrons and holes injected into each of well layers adjacent to a barrier layer interposed therebetween are also distributed according to the probability distributions thereof, and there is a possibility of transition of the electrons and the holes into adjacent well layers as well as the well layer into which the electrons and the holes are directly injected. The probability distributions of the electrons and the holes in the adjacent well layers may probabilistically overlap each other, and a thinner thickness of the barrier layer may result in a higher degree of overlapping between the probability distributions of the electrons and the holes in the adjacent well layers. A phenomenon in which the probability distributions of the electrons and the holes included in the adjacent well layers overlap each other is referred to as an overlap of the probability distributions.
A higher degree of overlap of the probability distributions causes an accordingly high possibility of transition of electrons and holes into adjacent well layers, and thus, likelihood of recombination of electrons and holes is lowered, thereby reducing internal quantum efficiency. Accordingly, in order to improve internal quantum efficiency, the barrier layer must have a sufficient thickness or a high band gap to block transition of electrons and/or holes into adjacent well layers.
In the related art, the barrier layer may be formed to have a certain thickness in order to block transition of electrons and holes into adjacent well layers. That is, the thickness of the barrier layer is set to be greater than or equal to a thickness at which the probability distributions of electrons and holes of well layers adjacent to the barrier layer do not overlap each other. The thickness of the barrier layer at which the probability distributions of electrons and holes of the well layers adjacent to the barrier layer do not overlap each other may be referred to as a skin depth of the barrier layer. The skin depth of the barrier layer is lowered as a difference in band gap between the well layer and the barrier layer increases and the thickness of the well layer increases. For example, in an active area having a structure wherein a GaN barrier layer is formed on an InGaN well layer containing 15% In and a thickness of 2 nm to 3 nm to emit light having a wavelength of about 460 nm to about 440 nm, since the barrier layer has a skin depth of about 5 nm when a difference in conduction band energy between the well layer and the barrier layer is 370 meV, the well layer may have a thickness of 10 nm to 15 nm.
As such, in the related art, since the thickness of the barrier layers may be greater than or equal to the skin depth of the barrier layers, the barrier layers 117b have a thick thickness. Accordingly, the barrier layers acts as blocking barriers in transfer of electrons and holes into each of the well layers 117w. Thus, the drive voltage of the light emitting diode is increased and electrons and holes are unevenly injected into the well layers, thereby causing deterioration in internal quantum efficiency.
Therefore, there is a need for development of a light emitting diode that includes an active area in which barrier layers have a greater thickness and a high band gap.